In the prior art, PS plates having a construction in which a lipophilic photosensitive resin layer is provided on a hydrophilic support have been broadly used as lithographic printing plates, and desired printing plates have been obtained generally a plate making method in which a masking exposure (surface exposure) is carried out via a lith film and then non-image parts are dissolved and removed.
In recent years, digitalization techniques in which image information is electronically treated, accumulated and output using a computer have been widely popularized, and various corresponding new image output systems have been put into practical use. As a result, there is a demand for a computer to plate (CTP) technique for directly producing a printing plate without mediating a lith film by scanning a high directivity light such as laser beam according to the digitized image information, and production of the printing plate precursors suited for such a technique therefore is now an important technical subject.
As one of the methods for obtaining such lithographic printing plates capable of effecting scanning exposure, a construction in which a photopolymerization system composition having excellent sensitizing speed is used as an ink-acceptable photosensitive resin layer (to be referred to as "photosensitive layer" hereinafter) to be provided on a hydrophilic support has been proposed and is already on the market. A precursor of such a construction renders possible simple and easy developing treatment and has desirable machine plate making and printing properties such as excellent resolution, inking property, printing durability and anti-scumming property.
The aforementioned photopolymerizable composition basically comprises an ethylenic unsaturated compound, a photopolymerization initiation system and a binder resin, and the image formation is effected by a process in which the photo-initiation system absorbs light to form an active radical which induces addition polymerization of the ethylenic unsaturated compound to cause insolubilization of the photosensitive layer. Most of the prior art proposals regarding photopolymerizable compositions capable of effecting scanning exposure disclose the use of photo-initiation systems having excellent photosensitivity and are described for example by Bruce M. Monroe et al. in Chemical Review, 93, 435 (1993) and by R. S. Davidson in Journal of Photochemistry and Biology A: Chemistry, 73, 81 (1993).
Regarding the background art CTP system which uses a photopolymerizable composition comprising such an initiation system and a long wavelength visible light source such as Ar laser (488 nm) or FD-YAG laser (532 nm) as the light source, writing at a more higher speed is expected for the purpose of improving productivity of the plate making step, but such a purpose has not been achieved yet because of insufficient output of the light source and insufficient sensitivity of the photosensitive material.
On the other hand, a semiconductor laser which uses, for example, an InGaN system material and can effect continuous oscillation at a range of from 350 nm to 450 nm has been put into practical steps in recent years. In the scanning exposure system which uses these short wavelength light sources, the semiconductor laser can be produced with low cost due to its structure, so that it is possessed of an advantage in that an economical system having sufficient output can be constructed. In addition, in comparison with the prior art system which uses FD-YAG or Ar laser, it renders possible the use of a photosensitive material which has a short wavelength photosensitive range and can be handled under a brighter safe light.
However, no information is available to date concerning a photo-initiation system having sufficient sensitivity for scanning exposure at a short wavelength region of from 350 nm to 450 nm.
In addition, as is described for example by J. P. Faussier in "Photoinitiated Polymerization--Theory and Applications": Rapra Review vol. 9, Report, Rapra Technology (1998) and by M. Tsunooka et al. in Prog. Polym. Sci., 21, 1 (1996), construction of a photoinitiation system having high sensitivity is a technique still greatly expected in the imaging field. Since a photoinitiation system comprising a sensitizing dye and an activator can generate an acid or base in addition to the aforementioned active radical, by selecting the activator, it is used for example in the image formation such as optical image formation, holography or color hard copy, in the field of electronic material production such as of photo resists and in photo-curing resin materials such as ink, paints and adhesives. In these industrial fields, there is a demand for finding a sensitizing dye having excellent light absorption ability and sensitizing ability, in order to induce decomposition of the activator efficiently.